African trypanosomes have the ability to rapidly and repeatedly alter their antigenic properties during infection of the mammalian host. Antigenic variation is associated with the sequential expression of different surface glycoproteins, thus providing the parasite with a mechanism for evading destruction by the host immune system. We will study the molecular basis of variant antigen expression in Trypanosoma (Trypanazoon) brucei brucei (1) by the biochemical and immunological characterization of surface antigens, (2) by examining the genomic organization of variant antigen genes and determining the relationship between structural and genomic variant antigen gene sequences, and (3) by studying the synthesis, processing and secretion of variant antigen. An understanding of the genetic basis of antigenic variation and the development of well-characterized immunological and biochemical probes of the variant surface glycoproteins will be important factors in the design of disease control strategy and the eventual analysis of regulatory aspects of variant antigen synthesis during the trypanosome life cycle.